Differentialy coherent phase-shift keying (DPSK) is sometimes employed as one form of PSK modulation system, and is herewith taken as an example for better understanding of the invention.
There is often used a delay detection system to demodulate DPSK modulated signals. In this system, DPSK demodulation is effected by multiplying a non-delayed signal and a delayed signal by a time corresponding to one symbol. This system requires a considerably accurate phase coincidence between carriers of the both input signals. Due to this, an accurate and stable delay line or other corresponding means must be used. However, it is extremely difficult to fabricate delay lines with an accurate desired delay time and not liable to changes in characteristics with temperature and time.
FIG. 1 is a block diagram showing a general construction of a DPSK demodulator of a delay detection type. Reference numeral 1 refers to a DPSK signal input terminal, 2 refers to a one-symbol delay line, 3 refers to a mixer, 4 refers to a low pass filter, 5 refers to a discriminator and 6 refers to an output terminal. FIG. 2 shows various signal waveforms. FIG. 2a shows the waveform of a carrier, FIG. 2b is of a data, FIG. 2c is of a signal obtained by two phases DPSK demodulation from the waveforms of FIGS. 2a and 2b and applied to the DPSK signal input terminal of FIG. 1. Namely, the two phases DPSK signal of FIG. 2c is obtained by inverting the carrier phase of FIG. 2a if the data phase is 1 and maintaining the original carrier phase if the data phase is 0. FIG. 2d is the waveform of an output from the one symbol delay line 2 of FIG. 1. FIG. 2e is the waveform of an output from the mixer 3, and FIG. 2f is the waveform of an output from the low pass filter 4. FIG. 2g is the waveform of a data demodulated by use of a signal which is discriminated by the discriminator 5 of FIG. 1. In brief, the data is demodulated by obtaining the product between the non-delayed DPSK signal applied to the system of FIG. 1 and the one symbol delayed signal therefrom. The proper data demodulation, however, cannot be expected as shown in FIGS. 3c, 3d, 3e and 3g unless the delay time of the delay line 2 is accurate. FIGS. 3c, 3d, 3e and 3g show waveforms corresponding to but deviated from those of FIGS. 2c, 2d, 2e and 2g, respectively. If the carrier frequency is high, the phase difference between the carriers of the non-delayed and delayed signals applied to the mixer 3 becomes large with a slight error of the delay time, thereby demodulating an improper data. Therefore, a high frequency of a carrier, in particular, requires an accurate and stable delay time.
To overcome this problem of carrier phases, there is proposed a system which removes the carrier components before such DPSK demodulation and thereafter demodulates the data by a baseband transmission system, as shown in FIG. 4. In the Figure, reference numerals 7, 8 and 14 refer to mixers, 9 refers to a 90.degree. phase shifter, 10 refers to a voltage control oscillator, 11 and 12 refer to low pass filters, 13 refers to a loop filter, and 15 refers to a DPSK demodulator. The circuit illustrated includes a well known Costas loop. The Costas loop demodulates the two phases modulated signal, and the DPSK demodulator 15 demodulates the output (baseband) from the loop. The system of FIG. 4, however, includes the closed loop which requires synchronization between the phases of the carrier and of the reference signal (output from the voltage control oscillator 10). As is widely acknowledged, such a phase synchronization is not so easy.